Whether you invest on your own or seek professional help, you should also be able to articulate your goals. Create a written plan to understand what you want and need your money to do for you, she says.

The rest involves timeless advice, including these four tips.

Diversify globally. This can be done by investors at any age, but it's especially advantageous for millennial investors who have more time to benefit from a globally diversified portfolio.

"Too many investors focus just on U.S. domestic funds and do not take advantage of the international and emerging market investment opportunities," says Ron Kloth, wealth manager at Dynamic Wealth Advisors in Tempe, Arizona.

Many investors make the mistake of overlooking international investments because they find it easier to follow what is happening domestically. But that can be a big mistake.

"The historical returns of international and emerging market funds have been higher than U.S. domestic funds of the same asset classes," so investors who diversify more overseas should be able to boost their overall portfolio return, Kloth says. "Even a 2 percent difference in return for a millennial investor can mean a big difference over a 30-to-40-year time span."

Let's say you invest $100 for 35 years at an 8 percent rate of return — the approximate historical return of U.S. stocks. The money would grow to $1,478 and would almost double to $2,810 over the same time frame if you increased your return just 2 percent, Kloth says.

Diversification also extends to the type of holdings. Different asset classes can help level out your rate of return over time by mitigating performance swings of bull markets, where prices are rising, and bear markets, where prices are falling, to make for a smoother ride. For example, if you invest in two different types of large-cap mutual funds, they will likely have a lot of crossover, Kloth says. Instead, spread your investments over a number of different asset classes including large-, mid- and small-cap stocks or mutual funds, as well as international and emerging markets.

Be sure to include a mix of growth, blend or value assets. They should be coupled with fixed-income options such as U.S. Treasury, municipal, and investment-grade or high-yielding corporate bonds that can be subdivided by the length of time to maturity, whether short, medium or long-term.

"Investing without diversification is like driving a car without shock absorbers," he says. "It can be a bumpy road."

Don't chase performance. Forget trying to time the market because it doesn't work. "Too often, investors chase returns, moving from one investment to another," and it's a "loser's game," says Robert Johnson, president and CEO for The American College of Financial Services in Bryn Mawr, Pennsylvania. "Investors often fall prey to their behavioral biases – becoming concerned and selling out of the market after the market has fallen, only to re-enter the market after it has turned around."

Another problem is rearview mirror investing or moving your current investments into last year's top-performing stocks or mutual funds. It's foolish, Kloth says, but it's a practice many investors use when rebalancing their assets.

"Your odds of going from a top-performing to an underperforming fund are greater when you chase performance," Kloth says. "Don't make investment choices by looking at what just happened."

Ask questions. Even a savvy investor needs professional help occasionally, but shop around for the right financial advisor. Ask prospective candidates how they get paid, how many clients they have and for how long they've had them, as well as what the average size portfolio is for the average client, Schleif says.

Get a sense of what kind of relationship you might have with the person by asking why you should hire the advisor and how this person will communicate with you on an ongoing basis.

Then probe a little deeper to find out about the advisor's investment philosophy and the firm's resources, including its due diligence of recommended investments and the oversight mechanisms in place to ensure that an advisor is investing your portfolio according to your wishes.

Michelle Hung, founder of the Sassy Investor and a fee-only financial advisor in Toronto, says if an advisor creates a portfolio for you, ask why those assets were selected for you.

"Everyone has their own interests at heart," Hung says. "So if something does not feel right, make sure to ask, no matter how silly you feel, especially if the advisors are paid a commission on the products they sell."

Be actively involved. Even when you find an advisor you trust, don't be disengaged. Too many investors blindly hand over their hard-earned money to strangers, investment advisors, hedge fund managers and financial planners to make decisions based on their "best interests," says Karl Kaufman, founder and CEO of financial membership service American Dream Investing in Boca Raton, Florida.

But it's essential that investors learn how to manage their own investments. "Claiming you don't have the time or interest in managing your investments can result in mediocre or worse returns," he says. "The opportunity costs of not investing wisely can mean delayed financial freedom and independence."